1. Field of the Invention
The present invention relates to a tilt detecting apparatus for and a tilt detecting method of detecting the amount of disc tilt (hereinafter merely referred to as the “amount of tilt”) in an optical disc, such as a DVD-ROM, a DVD-RW and a DVD-RAM, a tilt correcting apparatus for and a tilt correcting method of making a tilt correction on the basis of the detected amount of tilt, an information reproducing apparatus, an information recording apparatus and an information recording/reproducing apparatus, such as a DVD player and a DVD recorder, including such a tilt correcting apparatus.
2. Description of the Related Art
On an information reproducing apparatus, such as a DVD player, it is extremely important to irradiate an optical disc which is set, such as a DVD, with laser light for reading, from a direction perpendicular to the recording surface of the optical disc, in order to optically read information, accurately. This is because the inclination, i.e., “tilt” of the information recording surface of the optical disc, such as a DVD, to the laser light deteriorates a reproduction signal, depending on the amount of tilt (i.e., a tilt amount or a tilt angle) as being the extent of the inclination, or the angle of tilt. Thus, conventionally, on a read-only optical disc, such as a DVD-ROM, the amount of tilt of the information recording surface is detected by using the amplitude of a Radio Frequency (RF) signal, which is detected in accordance with the presence of record information (i.e. recording pits).
Moreover, the amount of tilt is not constant in the plane of an information recording surface, so that the amount of tilt is detected at a plurality of reading points which has a different radial direction position, in order to obtain the in-plane distribution of the amount of tilt. As described above, obtaining the in-plane distribution of the amount of tilt is referred to as a “disc profile measurement” or simply as a “profile” or “profiling”. Then, at the time of actual reproduction of the record information, the irradiation angle of the laser light is fine-tuned at each reading position so as to cancel the amount of tilt (which is referred to as a “tilt correction” herein as occasion demands) with respect to the information recording surface; namely, the laser irradiation angle to the information recording surface is set so as to increase the amplitude of the RF signal, to thereby prevent the deterioration of the reproduction signal by tilt.
On the other hand, even an information recording apparatus or an information recording/reproducing apparatus, such as a DVD recorder, at the time of recording, it is extremely important to irradiate an optical disc with laser light for writing, from a direction perpendicular to the information recording surface of the optical disc, in order to optically write information. This is because the presence of tilt causes a large light spot to be formed on the information recording surface, depending on the amount of tilt, to thereby deteriorate a writing signal. In this case, if the writable disc, such as a DVD+RW, is unrecorded, it is impossible to detect the RF signal corresponding to the record information, as in the above-mentioned DVD-ROM or the like. Thus, in place of the RF signal, the amplitude of a tracking error signal which uses land/groove tracks is used for the detection of the amount of tilt. For example, in reproducing the information on an optical disc in a wobble land pre-pit manner, the vertical asymmetry of a RF difference signal (i.e. the tracking error signal) and the vertical asymmetry of a land pre-pit signal can be used, to thereby detect the amount of tilt (e.g. refer to Japanese Patent Application Laid Open NO. 2002-288859).
Moreover, even for a two-layer disc or a multilayer disc in which two or a plurality of recording layers are laminated, a technique of performing such a tilt detection on any of the recording layers (hereinafter referred to as a “related art”, as occasion demands) has been proposed. According to the related art, the tilt detection can be performed for a desired recording layer, by removing stray light due to reflection from another recording layer out of the plurality of recording layers (e.g. refer to Japanese Patent Application Laid Open NO. Hei 9-320084).
According to the findings of the inventors of the present invention, the disc profile in the two-layer disc or the multilayer disc is normally not the same among the plurality of recording layers. In other words, the thickness of the recording layer is not constant or not uniform in the plane of the disc.
Therefore, only the tilt detection for the desired recording layer out of the plurality of recording layers, as in the above-mentioned related art, is not enough to make an optimum tilt correction for each of the plurality of recording layers. Moreover, in the above-mentioned related art, it is assumed that the disc profile measurement is performed for one recording layer, by focusing laser light on the one recording layer of the two-layer disc and that the disc profile measurement is performed for the other recording layer, by focusing the laser light on the other recording layer. If a time required for the disc profile measurement for one recording layer is a time “Δt”, which is about several seconds, for example, then, a time required for the disc profile measurement for a plurality of “n” recording layers is Δt×n or more, for example. Furthermore, in the disc profile measurement for one recording layer, normally, the tilt detection is performed at a plurality of measurement points while an optical pickup is displaced from the inner to the outer circumferential side of the optical disc. Thus, after all, the time required for the disc profile measurement for the plurality of “n” recording layers is Δt×n+“a time required for returning the optical pickup from the outer to the inner circumferential side”×n or more. Namely, if the disc profile measurement on the two-layer disc or the multilayer disc is performed for each recording layer, it takes time in proportion to the total number “n” of the recording layers, or it takes longer than proportional to the total number “n”. As described above, in the two-layer disc or the multilayer disc, there is such a technical problem that it is extremely difficult to perform the tilt detection and the disc profile measurement, accurately and quickly.